1. Field of the Invention
The present invention relates to the field of stress testing carried out on a rock sample taken from a formation, such analyses making it possible to compare the properties and characteristics of the formation. It applies more particularly to a cell for the triaxial testing under stress of a sample and to the testing method using such a cell.
2. Description of the Prior Art
The devices used at the present time for the triaxial testing of rocks comprise a closed enclosure, in which the rock sample to be tested is introduced, this sample being then subjected to two types of stresses, a vertical stress exerted by a piston on the upper section of the rock and lateral stresses, exerted by a pressurized fluid in the enclosure via an impermeable resilient membrane.
In order to evaluate the deformation of the sample, stress gauges are bonded thereto and are connected by transmission wires to measuring apparatus situated outside the enclosure.
Such a device for the triaxial testing of samples has been used for a number of years in rock mechanics applications, but although the results of such measurements are satisfactory for the analyses developed, the use of the device is more problematic. Such a rock testing device is divulged particularly in the U.S. Pat. No. 3,975,950.
In fact, the preparation is long and often unsuitable. It is first of all necessary to fix the stress gauges to the sample, to introduce therein a tubular membrane of small thickness and to connect the gauges by wires passing through this membrane.
This preparatory step finished, the sample is placed in the enclosure of the testing cell and is held therein by the piston bearing on the upper edge of the sample. Then a confinement fluid is injected about the membrane until the desired confinement pressure is obtained. The membrane in contact with this pressurized fluid then transmits lateral stresses to the rock.
The progressive force exerted by the piston is transmitted through the rock in the form of increasing deformations detected by the stress gauges, the stress exerted by the piston being able to go up to rupture of the structure of the sample.
This type of installation for triaxial testing has the manifest drawbacks of a long and inconvenient setting up phase. It is not always simple to empty the cell in order to withdraw the fractured sample and to uncouple the electric connections when the liquid may still be present on the tubular membrane. In addition, the problems are even more numerous when the rock is appreciably saturated with water for, in this case, fixing of the stress gauges becomes excessively difficult, as well as the passage of the connecting wires through the membrane. Finally, a major drawback of such a device is that it detects the pinpoint deformations at the level of each stress gauge, whereas it is important to be able to detect an overall displacement of the sample. Such an overall displacement announces a more or less pronounced anomaly of the test, and forms the only possible measurement in the case of rupture of the sample, the stress gauges then becoming inactive because they are pinpoint.
The information concerning the deformation at the time of rupture is no longer available, in the case of stress gauges, whereas it plays an essential role in the analysis of the mechanism of rupture of the material.